In manufacturing can bodies and can lids used as beverage cans or the like, an aluminum sheet or an aluminum alloy sheet is used. Although aluminum exhibits excellent formability and excellent flavor keeping property as a can body material and can lid material, aluminum has a drawback that aluminum exhibits less corrosion resistance compared to a surface-treated steel material.
On the other hand, a chromium-phosphate-based surface treatment agent has been used for applying surface treatment to the aluminum sheet. A chemical conversion coating formed on the aluminum sheet by the chromium-phosphate-based surface treatment agent exhibits the excellent corrosion resistance peculiar to the film and also exhibits excellent corrosion resistance and adhesiveness after coating various resin-based coating materials. Accordingly, the aluminum sheet coated with the chemical conversion coating has been used in broad application fields of aluminum material such as an architectural material, a household electrical appliance, a fin material, a car evaporator or a beverage can material.
However, recently, from a viewpoint of protection of environment, there has been a demand for a non-chromium-based surface treatment agent which can impart high corrosion resistance and adhesiveness compatible to the corrosion resistance and adhesiveness of the chromium-phosphate-based surface treatment agent.
As the non-chromium surface treatment agent used for forming the beverage can body, for example, the non-chromium surface treatment agent of a system which uses zirconium or titanium compound and phosphate compound. However, the chemical conversion coating formed of these systems exhibits less corrosion resistance and adhesiveness after coating compared to the film formed using the chromium-phosphate-based surface treatment agent and hence, the chemical conversion coating is not used in broad applications.
For example, JP-A-10-46101 (patent document 1) discloses aluminum-use surface treatment agent containing a phosphate-based compound and/or an organosilicon compound and organic compound polymer. However, the surface treatment agent used in such a technique leads to insufficient adhesiveness between a metallic material and a coating material and insufficient corrosion resistance of a coating material.
Further, JP-A-2000-6967 (patent document 2) and JP-A-2000-6979 (patent document 3) disclose a polyester resin coated aluminum seamless can having a composite formed coated film made of phosphoric acid or zirconium phosphate and an organic resin. However, in such a technique, when an aluminum alloy sheet material on which a thermoplastic resin layer is formed is formed by molding by applying strict forming such as deep drawing or drawing and ironing after the formation of the composite formed coated film, the formed can cannot acquire satisfying corrosion resistance and satisfying adhesiveness with a thermoplastic resin layer.
JP-B-56-33468 (patent document 4) discloses an aluminum-use surface treatment agent containing zirconium and/or titanium, phosphate and a fluoric material. However, the surface treatment agent used in such a technique leads to insufficient adhesiveness between a metallic material and a coating material and insufficient corrosion resistance of a coating material.
JP-B-63-30218 (patent document 5) discloses a non-chromium-based surface treatment agent containing water-soluble titanium and/or a zirconium compound, tannin and/or water-soluble or water-dispersing polymer. However, such a non-chromium-based surface treatment agent leads to insufficient corrosion resistance of a coating material.
JP-A-2002-275648 (patent document 6) discloses a metal surface treatment agent containing a water-soluble zirconium compound, a water-soluble or water-dispersing acrylic resin, and a water-soluble or water-dispersing thermosetting cross-linking material.
JP-A-2003-239079 (patent document 7) discloses a metal-surface chemical conversion treatment agent containing zirconium ion and an organic phosphoric acid compound, and pH of the water solution is set to equal to or larger than 1.8 and equal to or smaller than 4.0. However, a formed can formed by using either one of these surface treatment agents cannot acquire sufficient corrosion resistance and sufficient adhesiveness.    [Patent document 1] JP-A-H10-46101    [Patent document 2] JP-A-2000-6967    [Patent document 3] JP-A-2000-6979    [Patent document 4] JP-B-S56-33468    [Patent document 5] JP-B-S63-30218    [Patent document 6] JP-A-2002-275648    [Patent document 7] JP-A-2003-239079